1. Field of the Invention
This invention relates to photoresists and more particularly, to the application of photo-sensitive coatings to substrates such as circuit board base materials used in the production of printed circuit boards and the like.
2. Description of the Prior Art
Photosensitive or light-sensitive resists are thin coatings which, when exposed to light of the proper wavelength, are chemically changed in their solubility characteristics to certain solvents or developers. Two types are available, negative-acting and positive-acting resists. Prior to exposure, negative-acting resists are soluble in developer, but with light exposure, undergo chemical change and become insoluble in said developer. Exposure is done through a film pattern and the unexposed photoresist is selectively dissoved, softened or washed away, leaving the desired resist pattern on a substrate. Positive-acting resists work in the opposite manner, light exposure making the resist soluble in developer. The resist pattern that remains after development (and post-baking in some cases) is insoluble and chemically resistant to cleaning, plating and etching solutions used in processes of this nature. Typical examples of positive-acting photosensitive materials are the naphthoquinone-(1,2)-diazide sulfonic acid esters disclosed in U.S. Pat. No. 3,046,118. Other photosensitive materials are known in the art.
Processes involving the formation of a coating of photoresist such as in the formation of printed circuit boards, photolithographs, nameplates and the like, are known. In one process for the formation of a printed circuit board, a metal clad base sheet is coated with a photoresist composition and the photoresist is exposed through a positive or negative of the desired image. The light-exposed areas of the resist are rendered soluble by exposure and then are washed with a developer to leave the underlying metal layer exposed. An etchant in which the resist is impervious may be used to etch away the exposed metal, or selective plating or other processes may be accomplished and there remains a layer in the desired image pattern. The remaining resist may or may not be removed as desired.
In connection with processes for making printed circuit boards is the provision for and plating of through-holes. These holes extend between opposite surfaces of the base sheet and are used to form an electrical connection between said surfaces. Typically, they may be catalyzed and plated with an electroless plating solution.
The surface coating of the resist necessary for the above process has in the past been applied to surface base materials in liquid form using squeegees, rollers or wicks, by dipping, by spraying or wiping, or the like, to form a layer of liquid resist on the base material followed by solidification of the resist. A number of drawbacks attend these liquid methods of application. For example, the resist in liquid form is frequently forced into the through-holes where it (a) may not become sufficiently exposed to become soluble, or (b) may not be dissolved in a reasonable time. In either event, the presence of a residuum of resist in the through-holes prevents copper plating therein.
Plugging of through-holes is avoided by the process of U.S. Pat. No. 3,629,036, incorporated herein by reference, where a dry laminate is provided comprising a layer of photoresist and a backing substrate therefor. The materials of the photoresist and the backing layer are selected so that the resist is insoluble in a solvent for the backing. The laminate with the photoresist layer face downward is adhered to a base material, preferably, a circuit board base material, which may either be clad with a metal such as copper or unclad and the backing layer is washed from the composite so formed with a solvent that will not solvate or affect the photoresist layer, thereby leaving a uniform layer of photoresist adhered to the substrate, but not forced into the through-holes. In a preferred embodiment, the laminate comprises a water insoluble photoresist material and a water soluble backing, preferably a water soluble paper comprising carboxymthyl cellulose.
An additional technique for applying photoresist layers is disclosed in U.S. Pat. No. 3,469,982 where a resist is first coated as a film on a backing sheet and while still on the backing sheet, the film is adhered by means of heat and/or pressure to a substrate. The backing sheet may be transparent and the film may be exposed through the sheet. Before developing the photoresist, the backing sheet is cut to size and removed by stripping it from the photoresist layer. Though this technique solves some of the problems heretofore mentioned, such as plugging of through-holes, and it provides a sufficiently thick coating of photoresist, it introduces additional problems and disadvantages of its own.
One example of a difficulty with the technique of said U.S. Pat. No. 3,469,982 is that the backing layer must be stripped or torn from the photoresist layer. Therefore, the physical characteristics of the photoresist layer become critical. For example, it has been found that thick photoresist layers, i.e. one mil or more, are desirable to avoid wrinkling during application and tearing of the layer upon stripping of the backing. This results in substantially greater cost, poorer resolution of developed images, and longer exposure time. Moreover, the photoresist film may not be brittle or it will fracture. This requires careful compounding of the light-sensitive compound with various compatible resin systems.
In addition to the above, since the backing sheet must physically support the photoresist layer, it must have requisite strength properties. Therefore, the backing sheet must have a relatively thick dimension, typically more than one mil in thickness or more. When the photoresist layer is exposed through the backing sheet, the light passing through the sheet becomes diffused or scatters due to the thickness of the sheet, and consequently, image resolution is adversely affected.
A further problem with the technique of said patent relates to cutting and stripping of the support layer. The film of photoresist and support layer is typically fed continously from a feed roll and adhered to the substrate material by heat and pressure. Since it serves as a protective layer and is not removed until development, it must be cut to the size of the substrate, typically before exposure, and then stripped from the substrate prior to development. This creates problems where automated equipment is used. It is especially difficult to strip the support layer as processing frequently secures the edges thereof to the substrate.